android多媒体编程--画画板

package com.example.huahuaban;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;

import android.net.Uri;
import android.os.Bundle;
import android.os.Environment;
import android.app.Activity;
import android.content.Intent;
import android.graphics.Bitmap;
import android.graphics.Bitmap.CompressFormat;
import android.graphics.BitmapFactory;
import android.graphics.Canvas;
import android.graphics.Color;
import android.graphics.Matrix;
import android.graphics.Paint;
import android.graphics.PorterDuff.Mode;
import android.graphics.PorterDuffXfermode;
import android.view.MotionEvent;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.View.OnTouchListener;
import android.widget.ImageView;

public class MainActivity extends Activity implements OnClickListener{
    private ImageView img;
    Bitmap bitmapcopy;
    Paint paint;
    Bitmap bitmapsrc ;
    float startx =0;
    float starty = 0;
    private Canvas canvas;
    private Matrix mt;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        img = (ImageView) findViewById(R.id.img);
        findViewById(R.id.red).setOnClickListener(this);
        findViewById(R.id.gereen).setOnClickListener(this);
        findViewById(R.id.blue).setOnClickListener(this);
        findViewById(R.id.shua).setOnClickListener(this);
        findViewById(R.id.qingkong).setOnClickListener(this);
        findViewById(R.id.save).setOnClickListener(this);
        bitmapsrc = BitmapFactory.decodeResource(getResources(), R.drawable.bg);
        bitmapcopy = Bitmap.createBitmap(bitmapsrc.getWidth(), bitmapsrc.getHeight(), bitmapsrc.getConfig());
        //画笔
        paint = new Paint();
        canvas = new Canvas(bitmapcopy);
        mt = new Matrix();
        canvas.drawBitmap(bitmapsrc, mt, paint);
    
        img.setOnTouchListener(new OnTouchListener() {
            
            @Override
            public boolean onTouch(View arg0, MotionEvent event) {
                int action = event.getAction();
                
                switch (action) {
                case MotionEvent.ACTION_DOWN:
                    //设置线段的开始坐标
                    startx = event.getX();
                    starty = event.getY();
                    break;
                case MotionEvent.ACTION_MOVE:
                    //结束坐标
                    float x =  event.getX();
                    float y =  event.getY();
                    
                    canvas.drawLine(startx, starty, x, y, paint);
                    img.setImageBitmap(bitmapcopy);
                    //把结束坐标的值赋给开始坐标
                    startx = x;
                    starty = y;
                    break;
                case MotionEvent.ACTION_UP:
                    
                    break;

                }
                return true;
            }
        });
    }
    @Override
    public void onClick(View v) {
        switch (v.getId()) {
        case R.id.red:
            paint.setColor(Color.RED);
            break;
        case R.id.gereen:
            paint.setColor(Color.GREEN);
            break;
        case R.id.blue:
            paint.setColor(Color.BLUE);
            break;
        case R.id.qingkong:
             paint.setXfermode(new PorterDuffXfermode(Mode.CLEAR));  
             canvas.drawBitmap(bitmapsrc, mt, paint);
             paint.setXfermode(new PorterDuffXfermode(Mode.SRC)); 
             img.invalidate();
            break;
        case R.id.shua:
            paint.setStrokeWidth(3.5f);
            break;
        case R.id.save:
            File file = new File("sdcard/ok1.png");
            FileOutputStream out = null;
            try {
                out = new FileOutputStream(file);
            } catch (FileNotFoundException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            //保存文件，参数分别为格式，压缩质量0-100，输出流
            bitmapcopy.compress(CompressFormat.PNG, 100, out);
            //发送更新多媒体数据库的广播
            Intent intent = new Intent();
            intent.setAction(Intent.ACTION_MEDIA_MOUNTED);
            intent.setData(Uri.fromFile(Environment.getExternalStorageDirectory()));
            sendBroadcast(intent);
            break;

        
        }
        
    }

}

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
  
    tools:context=".MainActivity" 
    android:orientation="vertical">

    <ImageView
        android:id="@+id/img"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
       
        android:src="@drawable/bg"/>
    <LinearLayout 
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        >
        <Button 
            android:id="@+id/red"
            android:background="#FF0000"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            
            />
        <Button 
            android:id="@+id/gereen"
            android:background="#00FF00"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            
            />
        <Button 
            android:id="@+id/blue"
            android:background="#3366FF"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            
            />
        <Button 
            android:id="@+id/shua"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="加粗"
            />
        <Button 
            android:id="@+id/qingkong"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="清空"
            />
    </LinearLayout>
         <Button 
            android:id="@+id/save"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="保存"
            />

</LinearLayout>